AbstractThe objective of this work was to develop a practical method for monitoring the quality of oils during the frying process. A special effort was made to find a technique that would not be affected by dilution, since replenishment with fresh oil to varying degrees is a frequent necessity. Nine analytical methods, i.e., measurements of viscosity, polymers, change in dielectric constant, polar compounds, dimers, free fatty acids, smoke point, carbonyls and cyclic monomers, as well as certain combinations of these measurements were evaluated. Since each single method was influenced by replenishment with fresh oil, combinations of two methods were studied in an attempt to produce a single value unaffected by dilution. The ratio polymer/FOS (polymers according to Peled's technique of methylation and extraction and change in dielectric constant by Foodoil Sensor readings) proved not only to be adequate for monitoring the quality of the used oil, but also was affected minimally by replenishment.